Radiofrequency transmission/reception systems fed with differential electrical signals are very attractive for current and future wireless communications systems, in particular for the concepts of autonomous communicating objects. A differential feed is a feed by two signals of equal amplitude in opposite phase. It helps to reduce, or indeed to eliminate, undesirable so-called “common mode” noise in transmission and reception systems.
In the realm of mobile telephony for example, when a non-differential system is used, a significant degradation of the radiation performance is indeed observed when the operator holds a handset furnished with such a system. This degradation is caused by the variation, due to the operator's hand, of the distribution of the current over the chassis of the handset used as ground plane. The use of a differential feed renders the system symmetric and thus reduces the concentration of current on the casing of the handset: it therefore renders the handset less sensitive to the common mode noise introduced by the operator's hand.
In the realm of antennas, a non-differential feed gives rise to the radiation of an undesirable cross-component due to the common mode flowing around the non-symmetric feed cables. The use of a differential feed eliminates the cross-radiation of the measurement cables and thus makes it possible to obtain reproducible measurements independent of the measurement context as well as perfectly symmetric radiation patterns.
In the realm of active hardware components, the power amplifiers of “push-pull” type, whose structure is differential, exhibit several advantages, such as the splitting of the power at output and the elimination of the higher-order harmonics. At reception, low noise differential amplifiers exhibit much promise in terms of noise factor reduction. Hence, the use of a differential structure prevents the undesirable triggering of the oscillators by the common mode noise.
A differential bi-strip delay line can be useful for joining two differential devices, such as for example, two filtering devices, so as to form a higher-order filter. In the particular case of the joining of two filtering devices, the differential bi-strip delay line must have the characteristics of a quarter-wave (π/2) phase shift line so as to be able to be used as impedance inverter.
More generally, a differential bi-strip delay line can be useful in a large number of applications making it necessary to join differential devices, including in the guise of phase shifter. For example, in a feed application for an antenna array, where several different antennas are fed by one or more sources, at least one phase shifter of this type can advantageously be envisaged.
Now, more and more differential devices, such as filtering devices or dipole antennas, are being designed with differential CPS (“CoPlanar Stripline”) technology. Indeed, differential CPS technology makes it possible to profit from the advantages of differential structures while allowing simple coplanar integration with discrete elements: it is not necessary to create connections to link the elements together. Furthermore, the absence of any ground plane makes it possible to envisage a simple and less disturbing joining with, for example, a differential antenna.
It is therefore advantageous to also use this technology to produce a differential bi-strip delay line, in particular a quarter-wave line. According to this technique, a bi-strip line for propagating a differential signal comprises two rectilinear conducting strips disposed in parallel on one and the same face of a dielectric substrate and each comprising a first and a second end. The two first ends of the two conducting strips form two conductors of a first bi-strip port for connection to a first external differential device. The two second ends of the two conducting strips form two conductors of a second bi-strip port for connection to a second external differential device.
Thus, a differential bi-strip delay line designed in this way can be joined in an optimal manner to external devices designed with differential CPS technology. The delay that it induces and its impedance are directly related to its length, the separation between its two conducting strips and their width.
For example, the document “Broadband and compact coupled coplanar stripline filters with impedance steps”, by Ning Yang et al, IEEE Transactions on Microwave Theory and Techniques, vol. 55, No. 12, December 2007, describes the realization of a filter with differential CPS technology, in particular with reference to FIG. 12 of “Broadband and compact coupled coplanar stripline filters with impedance steps”. This compact topology makes it possible to attain high passbands with large out-of-band rejection for filters of order 2, 3 or 4. Unfortunately, the interposition of a differential CPS technology quarter-wave delay line between two filtering devices, such as that illustrated in the aforementioned document, although necessary to obtain a higher-order filter with good rejection properties, substantially increases the bulkiness of the complete device, mainly because of its length.
It may thus be desired to design, with differential CPS technology, a bi-strip delay line exhibiting better compactness while preserving the same performance in terms of phase shift and impedance matching as a bi-strip propagation delay line with predetermined phase shift.